The toll-like receptor (TLR) family plays a fundamental role in pathogen recognition and activation of innate immunity. Toll-like receptor 8 (TLR8) is predominantly expressed by myeloid immune cells and activation of this receptor stimulates a broad immunological response. Agonists of TLR8 activate myeloid dendritic cells, monocytes, monocyte-derived dendridic cells and Kupffer cells leading to the production of proinflammatory cytokines and chemokines, such as interleukin-18 (IL-18), interleukin-12 (IL-12), tumor necrosis factor-alpha (TNF-α), and interferon-gamma (IFN-γ). Such agonists also promote the increased expression of co-stimulatory molecules such as CD8+ cells, major histocompatibility complex molecules (MAIT, NK cells), and chemokine receptors.
Collectively, activation of these innate and adaptive immune responses induces an immune response and provides a therapeutic benefit in various conditions involving autoimmunity, inflammation, allergy, asthma, graft rejection, graft versus host disease (GvHD), infection, cancer, and immunodeficiency. For example, with respect to hepatitis B, activation of TLR8 on professional antigen presenting cells (pAPCs) and other intrahepatic immune cells is associated with induction of IL-12 and proinflammatory cytokines, which is expected to augment HBV-specific T cell responses, activate intrahepatic NK cells and drive reconstitution of antiviral immunity. See e.g. Wille-Reece, U. et al. J Exp Med 203, 1249-1258 (2006); Peng, G. et al., Science 309, 1380-1384 (2005); Jo, J. et al., PLoS Pathogens 10, e1004210 (2014) and Watashi, K. et al., J Biol Chem 288, 31715-31727 (2013).
Given the potential to treat a wide array of diseases, there remains a need for novel modulators of toll like receptors, for example TLR8. Potent and selective modulators of TLR8 that have reduced potential for off target liabilities are particularly desirable.